This invention relates to multilamp photoflash devices having circuit means for igniting the flashlamps and, more particularly, to high-voltage photoflash arrays with improved means for providing electrostatic protection.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. A currently marketed photoflash unit described in U.S. Pat. No. 3,894,226, and referred to as a flip flash, employs high-voltage type lamps adapted to be ignited sequentially by successively applied high-voltage firing pulses from a source such as a camera-shutter-actuated piezoelectric element. The flip flash unit comprises a planar array of eight high-voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The lamps are arranged in two groups of four disposed on the upper and lower halves respectively of the rectangular-shaped circuit board. A set of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps, while a set of terminal contacts at the top of the unit is operatively associated with the lower group of four lamps. The application of successive high-voltage pulses (e.g., 500 to 4,000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes the four lamps at the upper half of the array to be sequentially ignited. The array may then be turned end for end and again inserted into the camera in order to flash the remaining four lamps.
The flip flash circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit traces, including the terminal contacts, on one side. The flashlamp leads are electrically connected to these circuit traces by means of eyelets secured to the circuit board and crimped to the lead wires. The circuitry on the board includes six printed, normally opened, connect switches that chemically change from a high to low resistance, so as to become electrically conducting after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of these switches is to promote lamp sequencing and one-at-a-time flashing. The four lamps of each group are arranged in parallel with three of the four lamps being connected in series with their respective thermal connect switches. Initially, only the first of the group of four lamps is connected directly to the voltage pulse source. When this first group flashes, it causes its associated thermal connect switch (which is series connected with the next, or second lamp) to become permanently conductive. Because of this action, the second lamp of the group of four is connected to the pulse source. This sequence of events is repeated until all four lamps have been flashed.
The primers used in the high-voltage type flashlamps employed in such arrays are designed to be highly sensitive toward high-voltage breakdown. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamps. This high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezoelectric sources that are practical for incorporation into modern miniature cameras. The mechanical energy delivered to the piezoelectric crystal, and thereby the electrical output energy therefrom, is limited both by the size of the device and by the necessity to minimize camera vibration and motion during use.
The high degree of electrical sensitivity needed in high-voltage flashlamps gives rise to distinct problems of inadvertent flashing during handling of the array package. Any static charges on equipment and personnel can cause the lamps to flash. This problem is discussed in the aforementioned U.S. Pat. No. 3,894,226, and one means described therein for protecting against inadvertent flashing is to make the reflector member electrically conductive, such as fabricating it of metal or metal-coated plastic and electrically connecting the reflector to an electrical "ground" portion of the circuitry on the circuit board. Thus, the reflector member functions as an electrical shield and increases the stray capacitance to ground of the electrical "ground" of the circuitry, reducing the possibility of accidental flashing of lamps by electrostatic voltage charge on a person or object touching the array.
A further approach used in marketed flip flash arrays for providing electrostatic protection is to metalize the back surface of the circuit board and connect that metalized surface to the common circuit conductor run, for example, by means of an eyelet through the board, thereby providing a planar conductive shield behind the lamps and most of the circuitry.
Although the above-described protective packaging features are effective with respect to various sources of electrostatic discharges, we have observed that commercial arrays employing such shield elements are still subject to inadvertent electrostatic-caused flashing of the lamps contained therein. More specifically, testing has shown that the most susceptible mode involves imposition of a voltage gradient between the common circuit terminal contact and the exterior back face of the array. Depending upon the innate sensitivity of the lamps used, from about five percent to nearly fifty percent of all lamps tested in this mode may flash. A standardized test, referred to as a rear surface electrostatic flash test, uses a 10,000 volt DC pulse from a charged capacitor applied between the common circuit terminal and a metal plate contacting the exterior of the back cover of the array.